This invention relates to a piston for a swash plate type compressor used mainly in a vehicle air conditioning device.
The sliding portions on the piston outer surface and the cylinder inner surface are required to have good airtightness and wear resistancy, and various proposals were made in the past so as to meet these requirements.
As an example, it is already known to have the piston outer surface surrounded by a cup-shaped cover or sheath of fluorine resin consisting of ethylene fluoride or an ethylene fluoride-vinyl compound copolymer (hereafter designated simply as "fluorine resin"). Although such piston may thus be endowed with improved wear resistancy and sliding properties, the cylindrical portion of the sheath necessarily has increased thickness because the sheath is prefabricated in the cup-like shape and applied as it is to the piston. This sheath portion exhibits considerable thermal expansion, resulting in increased fluctuations in the dimension of the gap of piston or side clearance with changes in the temperatures. Thus the compressor may exhibit unstable performance due to considerable changes in the volume efficiency caused by changes in the operating states of the compressor.
It has also been proposed to directly apply the coating of the above fluorine resin to the piston outer surface, with the coating having reduced thickness of the order of 20 to 100 micro meters. While the piston side clearance may thus be reduced to thereby increase the volume efficiency of the compressor, it is necessary, during application of the coating or during surface treatment of the piston blank to be performed in advance of the coating application, such as shot peening, to apply a mask, e.g. by way of the adhesive tape, to the piston flank portion as well as the piston concave that are adapted to receive balls interposed between the piston and the swash plate. Moreover, in the course of the baking step for bonding the coating to the blank surface, surface zones of the piston blank devoid of such coating, that is, the spherical concave and flank portions, may be subjected to strain because these portions have reduced wall thicknesses as compared to the columnar portions on the both piston end portions and moreover may undergo more abrupt rise in the temperatures. Moreover, the coating material may be exfoliated occasionally starting from the boundary line between masked and unmasked zones, especially when the material is locally coagulated on such boundary line on the piston blank surface.